Synchronously-switched multi-input demodulating comparator

ABSTRACT

Methods and systems are described for obtaining a set of carrier-modulated symbols of a carrier-modulated codeword, each carrier-modulated symbol received via a respective wire of a plurality of wires of a multi-wire bus, applying each carrier-modulated symbol of the set of carrier-modulated symbols to a corresponding transistor of a set of transistors, the set of transistors further connected to a pair of output nodes according to a sub-channel vector of a plurality of mutually orthogonal sub-channel vectors, recovering a demodulation signal from the carrier-modulated symbols, and generating a demodulated sub-channel data output as a differential voltage on the pair of output nodes based on a linear combination of the set of carrier-modulated symbols by controlling conductivity of the set of transistors according to the demodulation signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/611,523, filed Dec. 28, 2017, naming Armin Tajalli, entitled “Combined Multi-Input Comparator and Demodulator”, which is hereby incorporated by reference in its entirety for all purposes.

REFERENCES

The following references are herein incorporated by reference in their entirety for all purposes:

U.S. Patent Publication No. 2011/0268225 of U.S. patent application Ser. No. 12/784,414, filed May 20, 2010, naming Harm Cronie and Amin Shokrollahi, entitled “Orthogonal Differential Vector Signaling”, hereinafter identified as [Cronie I];

U.S. patent application Ser. No. 13/030,027, filed Feb. 17, 2011, now U.S. Pat. No. 8,649,445, naming Harm Cronie, Amin Shokrollahi and Armin Tajalli, entitled “Methods and Systems for Noise Resilient, Pin-Efficient and Low Power Communications with Sparse Signaling Codes”, hereinafter identified as [Cronie II];

U.S. patent application Ser. No. 14/158,452, filed Jan. 17, 2014, now U.S. Pat. No. 9,124,557, naming John Fox, Brian Holden, Peter Hunt, John D Keay, Amin Shokrollahi, Richard Simpson, Anant Singh, Andrew Kevin John Stewart, and Giuseppe Surace, entitled “Chip-to-Chip Communication with Reduced SSO Noise”, hereinafter identified as [Fox I];

U.S. patent application Ser. No. 13/842,740, filed Mar. 15, 2013, now U.S. Pat. No. 9,300,503, naming Brian Holden, Amin Shokrollahi and Anant Singh, entitled “Methods and Systems for Skew Tolerance in and Advanced Detectors for Vector Signaling Codes for Chip-to-Chip Communication”, hereinafter identified as [Holden I];

U.S. Provisional Patent Application No. 61/934,804, filed Feb. 2, 2014, naming Ali Hormati and Amin Shokrollahi, entitled “Methods for Code Evaluation Using ISI Ratio”, hereinafter identified as [Hormati I];

U.S. Provisional Patent Application No. 62/026,860, filed Jul. 21, 2014, naming Ali Hormati and Amin Shokrollahi, entitled “Multidrop Data Transfer”, hereinafter identified as [Hormati II];

U.S. patent application Ser. No. 15/194,497, filed Jun. 27, 2016, now U.S. Pat. No. 9,832,046, naming Ali Hormati, Armin Tajalli, and Amin Shokrollahi, entitled “Method and Apparatus for High-Speed Chip-to-Chip Communications”, hereinafter identified as [Hormati III];

U.S. patent application Ser. No. 15/802,365, filed Nov. 2, 2017, now U.S. Pat. No. 10,347,283, naming Ali Hormati and Armin Tajalli, entitled “Clock Data Recovery in Multilane Data Receiver”, hereinafter identified as [Hormati IV].

U.S. Provisional Patent Application No. 61/934,807, filed Feb. 2, 2014, naming Amin Shokrollahi, entitled “Vector Signaling Codes with High pin-efficiency and their Application to Chip-to-Chip Communications and Storage”, hereinafter identified as [Shokrollahi I];

U.S. Provisional Patent Application No. 61/839,360, filed Jun. 23, 2013, naming Amin Shokrollahi, entitled “Vector Signaling Codes with Reduced Receiver Complexity”, hereinafter identified as [Shokrollahi II].

U.S. Provisional Patent Application No. 61/946,574, filed Feb. 28, 2014, naming Amin Shokrollahi, Brian Holden, and Richard Simpson, entitled “Clock Embedded Vector Signaling Codes”, hereinafter identified as [Shokrollahi III].

U.S. Provisional Patent Application No. 62/015,172, filed Jul. 10, 2014, naming Amin Shokrollahi and Roger Ulrich, entitled “Vector Signaling Codes with Increased Signal to Noise Characteristics”, hereinafter identified as [Shokrollahi IV].

U.S. patent application Ser. No. 13/895,206, filed May 15, 2013, now U.S. Pat. No. 9,288,082, naming Roger Ulrich and Peter Hunt, entitled “Circuits for Efficient Detection of Vector Signaling Codes for Chip-to-Chip Communications using Sums of Differences”, hereinafter identified as [Ulrich I].

“Controlled Intersymbol Interference Design Techniques of Conventional Interconnection Systems for Data Rates beyond 20 Gbps”, Wendemagegnehu T. Beyene and Amir Amirkhany, IEEE Transactions on Advanced Packaging, Vol. 31 No. 4, pg. 731-740, November 2008, hereinafter identified as [Beyene].

TECHNICAL FIELD

The present invention relates to communications in general and in particular to the transmission of signals capable of conveying information and detection of those signals in wired communication.

BACKGROUND

In communication systems, a goal is to transport information from one physical location to another. It is typically desirable that the transport of this information is reliable, is fast and consumes a minimal amount of resources. Methods of information transport are broadly categorized into “baseband” methods that dedicate use of the physical communications channel to one transport method, and “broadband” methods that partition the physical communications channel in the frequency domain, creating two or more independent frequency channels upon which a transport method may be applied.

Baseband methods may be further categorized by physical medium. One common information transfer medium is the serial communications link, which may be based on a single wire circuit relative to ground or other common reference, multiple such circuits relative to ground or other common reference, or multiple such circuits used in relation to each other. A common example of the latter uses differential signaling (“DS”). Differential signaling operates by sending a signal on one wire and the opposite of that signal on a matching wire. The signal information is represented by the difference between the wires, rather than their absolute values relative to ground or other fixed reference.

Parallel data transfer is also commonly used to provide increased interconnection bandwidth, with busses growing from 16 or fewer wires, to 32, 64, and more. As crosstalk and noise induced on the parallel signal lines can produce receive errors, parity was added to improve error detection, and signal anomalies were addressed through active bus termination methods. However, these wide data transfer widths inevitably resulted in data skew, which became the limiting factor in increased bus data transfer throughput. Alternative approaches were developed utilizing narrower bus widths operating at much higher clock speeds, with significant effort placed on optimizing the transmission line characteristics of the interconnection medium, including use of impedance-controlled connectors and micro stripline wiring. Even so, the inevitable path imperfections required use of active equalization and inter-symbol interference (ISI) elimination techniques, including active pre-emphasis compensation for transmitters and Continuous Time Linear Equalization (CTLE) and Decision Feedback Equalization (DFE) for receivers, all of which increased the complexity and power consumption of the communications interface.

A number of signaling methods are known that maintain the desirable properties of DS, while increasing pin efficiency over DS. One such method is Vector signaling. With vector signaling, a plurality of signals on a plurality of wires is considered collectively although each of the plurality of signals might be independent. Thus, vector signaling codes can combine the robustness of single circuit DS and the high wire count data transfer throughput of parallel data transfer. Each of the collective signals in the transport medium carrying a vector signaling codeword is referred to as a component, and the number of plurality of wires is referred to as the “dimension” of the codeword (sometimes also called a “vector”). With binary vector signaling, each component or “symbol” of the vector takes on one of two possible values. With non-binary vector signaling, each symbol has a value that is a selection from a set of more than two possible values. The set of values that a symbol of the vector may take on is called the “alphabet” of the vector signaling code. A vector signaling code, as described herein, is a collection C of vectors of the same length N, called codewords. Any suitable subset of a vector signaling code denotes a “subcode” of that code. Such a subcode may itself be a vector signaling code. In operation, the coordinates of the codewords are bounded, and we choose to represent them by real numbers between −1 and 1. The ratio between the binary logarithm of the size of C and the length N is called the pin-efficiency of the vector signaling code. A vector signaling code is called “balanced” if for all its codewords the sum of the coordinates is always zero. Additional examples of vector signaling methods are described in Cronie I, Cronie II, Cronie III, Cronie IV, Fox I, Fox II, Fox III, Holden I, Shokrollahi I, Shokrollahi II, and Hormati I.

As previously described, broadband signaling methods partition the available information transfer medium in the frequency domain, creating two or more frequency-domain “channels” which may then may transport information in a comparable manner to baseband circuits, using known methods of carrier modulation to convert the baseband information into a frequency-domain channel signal. As each such channel can be independently controlled as to amplitude, modulation, and information encoding, it is possible to adapt the collection of channels to widely varying information transfer medium characteristics, including variations in signal loss, distortion, and noise over time and frequency.

Asymmetric Digital Subscriber Line or ADSL is one widely deployed broadband signaling method used to transport digital data over legacy copper telephony circuits. In ADSL, each of potentially several hundred frequency-domain channels is independently configured for amplitude, modulation method, and digital carrying capacity, based on the particular noise and loss characteristics of the copper circuit being used for transport.

BRIEF DESCRIPTION

Methods and systems are described for obtaining a set of carrier-modulated symbols of a carrier-modulated codeword, each carrier-modulated symbol received via a respective wire of a plurality of wires of a multi-wire bus, applying each carrier-modulated symbol of the set of carrier-modulated symbols to a corresponding transistor of a set of transistors, each transistor of the set of transistors connected to a respective output node of a pair of output nodes according to elements of a sub-channel vector, and controlling conductivity of the set of transistors according to a demodulation signal operating at a frequency recovered from the carrier-modulated symbols to responsively generate a demodulated sub-channel data output as a linear combination of the set of carrier-modulated symbols forming a differential voltage on the pair of output nodes.

Communication of digital information using a combination of baseband and broadband techniques over multiple wires is described. A four wire communications channel having 35 dB of attenuation at 37.5 GHz is used in provided examples as a typical transport medium for use with the systems and methods described herein. One embodiment creates two frequency-based channels over the transport medium, with each channel using a combination of a vector signaling code and duobinary encoding to transport sets of three data bits over four wires at an effective rate of 56 Gigabits per second per wire.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a block diagram of a receiver embodiment.

FIG. 2 is a schematic of one embodiment of a circuit detecting one ENRZ subchannel.

FIG. 3 is a schematic of another embodiment of a circuit detecting one ENRZ subchannel.

FIG. 4 is a schematic of one embodiment of a circuit detecting one carrier-modulated ENRZ subchannel.

FIG. 5 is a schematic of another embodiment of a circuit detecting one carrier-modulated ENRZ subchannel.

FIG. 6 is a schematic of a further embodiment of a circuit detecting one carrier-modulated ENRZ subchannel.

FIG. 7 illustrates waveforms for the carrier-modulated data encoding and decoding operations.

FIGS. 8A and 8B illustrate alternative embodiments for receiver clock generation.

FIG. 9 is a flowchart of a method, in accordance with some embodiments.

DETAILED DESCRIPTION

Interconnection has long been a limiting factor in the design of large digital systems. Whether at the level of modules interconnected by a backplane, or of functional subsystems interconnected within a large printed circuit board, the need for reliable, error free, high-speed digital interconnection has constantly pushed the limits of available technology to its limits.

The systems and methods described herein provide robust, reliable transfer of data between at least one transmitting device and at least one receiving device, at data rates of at least 50 Gigabits per second per interconnection wire. An example channel model having the frequency- and time-domain characteristics illustrated in FIG. 1 will be used. It will be obvious to one familiar with the art that such a transport channel is incompatible with conventional communication signaling methods; for example, straightforward NRZ signaling at an example 112 Gibabits/second has a Nyquist frequency of 56 GHz, corresponding to an intractable 46 dB attenuation over the proposed physical transport channel.

This proposed data rate also strains integrated circuit data processing capabilities within the attached transmitting and receiving devices. It is therefore presumed that high-speed data handling in these devices will be distributed across multiple parallel processing “phases”. As one example, rather than a single data path handling data at 100 Gigabits per second (i.e. with merely 10 picosecond between bits), the same data stream may be distributed across sixteen processing phases, each one thus having a more reasonable 160 picoseconds of processing time per bit. However, this added processing time comes at the cost of significantly increased complexity from the additional processing elements. This distribution of processing also can lead to increased latency before a given digital bit result becomes available, limiting the ability to utilize that result in predicting a subsequent bit result, which is the basis of the Decision Feedback Equalization or DFE method.

The increasing data transfer rates also lead to physical issues as the wavelength of the propagating signals on the interconnection shrinks. As one example, the propagating signal wavelength at 56 Gigahertz on a printed circuit micro stripline is approximately 4 millimeters, thus periodic anomalies with merely fractional wavelength dimensions (even including the weave of the impregnated fabric comprising the circuit board) may represent a significant disturbance to signal integrity, stressing available equalization and compensation methods.

Encoding Information Using Hadamard Transforms

As taught in [Cronie I], the Hadamard Transform, also known as the Walsh-Hadamard transform, is a square matrix of entries +1 and −1 so arranged that both all rows and all columns are mutually orthogonal. Hadamard matrices are known for all sizes 2N as well as for selected other sizes. In particular, the description herein utilizes the 4×4 Hadamard matrix as the example encoder.

The order 4 Hadamard matrix used in our examples is:

$\begin{matrix} {H_{4} = \begin{bmatrix} {+ 1} & {+ 1} & {+ 1} & {+ 1} \\ {+ 1} & {- 1} & {+ 1} & {- 1} \\ {+ 1} & {+ 1} & {- 1} & {- 1} \\ {+ 1} & {- 1} & {- 1} & {+ 1} \end{bmatrix}} & \left( {{Eqn}.\mspace{14mu} 1} \right) \end{matrix}$ and encoding of the three informational bits A, B, C may be obtained by multiplying those informational bits times the rows 2, 3, and 4 of the Hadamard matrix H₄ to obtain four output values, subsequently called “symbol values”. By convention, the results are scaled by an appropriate constant factor so as to bound the symbol values to the range +1 to −1. It may be noted that the first row of H₄ corresponds to common mode signaling, which is not used herein, with the next three vectors being used to encode bits A, B, and C respectively into outputs W, X, Y, Z, these vectors also being called “modes” or “subchannels” of the Hadamard code. As the encoded outputs simultaneously carry information derived from the encoding of A, B, and C, the outputs will be a superposition or summation of modes, i.e. a sum of the sub-channel code vectors of the vector signaling code.

One familiar with the art will note that all possible values of A, B, C encoded in this manner result in mode summed values for W, X, Y, Z which are balanced; that is, summing to the constant value zero. If the mode summed values for W, X, Y, Z are scaled such that their maximum absolute value is 1 (that is, the signals are in the range +1 to −1 for convenience of description,) it will be noted that all achievable values are permutations of the vector (+1, −1/3, −1/3, −1/3) or of the vector (−1, 1/3, 1/3, 1/3). These are called the codewords of the vector signaling code H4. As used herein, this H4 code will subsequently be called Ensemble NRZ code or ENRZ and will be used as a representative example of vector signaling code in subsequent examples, without implying limitation.

ENRZ

[Hormati I] teaches that ENRZ has optimum Inter Symbol Interference (ISI) characteristics, and [Holden I] and [Ulrich I] teach it is capable of efficient detection. As previously described, ENRZ encodes three binary data bits into a four-symbol codeword for transmission, as one example, over four wires of a transport medium. If ENRZ signaling is used over four wires of the proposed channel, the data transfer rate may be achieved with merely a 75 Gigasymbol/second signaling rate, equivalent to 112 Gbps per wire pair, for the two pair transport channel.

Duobinary Encoding

Duobinary encoding is a solution known in the art in which consecutive bits of a serially transmitted data stream are processed to shape and constrain the resulting transmit data spectrum. It is well known that Inter-Symbol Interference (ISI) such as may be produced by transmission medium perturbations will result in the received amplitude of a signal in one unit interval to be perturbed by residual energy from previous unit intervals. As one example, inverted pulse reflections from a perturbation of the transmission medium will cause a received signal to be reduced by the residual influence of previously transmitted signals. Thus, a transmitter informed of this effect might combine a presently transmitted signal value with that of a previous transmission, in an attempt to anticipate or pre-compensate for this inter-symbol interference effect. Thus, use of partial response codes such as duobinary are often described as a particular form of pre-equalization filtering intended to produce constructive ISI, rather than as a literal data encoding means.

As described in [Beyene], other partial-response codes are known to have comparable ISI management capabilities. For reference purposes, the characteristic equations defining these encodings or filterings are listed in Table I.

TABLE I Partial Response System Characteristic Equation Duobinary x_(n) + x_(n−1) Dicode x_(n) − x_(n−1) Modified Duobinary x_(n) − x_(n−2) Class 2 x_(n) + 2x_(n−1) + x_(n−2)

Unless otherwise described, as used herein the duobinary processing performed is assumed to be a summation of the present and immediately previous transmit unit interval signal, each scaled by a factor of 0.5. Optionally, this may be combined with a transmit lowpass filter to further control the transmit spectrum. In other embodiments, ISI-controlling encoding is combined in any order with Hadamard encoding, where the ISI-controlling encoding is any of duobinary, modified duobinary, dicode, class2, or a Hamming filter as subsequently described. In such embodiments, the ISI-controlling encoding may also be described as being performed by a partial response encoder, embodying any of the partial response encodings or filterings above.

If the characteristics of the communications channel are extremely well understood, it may be possible to configure the ISI-controlling operation of the transmitter such that no explicit complementary operation is required at the receiver, the effective action of the channel characteristics themselves serving to perform the inverse operation. Other embodiments may explicitly detect, as one example, the ternary signals produced by duobinary encoding of binary data, followed by an explicit duobinary to binary decoding operation. Alternatively, commonly used receiver ISI elimination techniques such as DFE will also efficiently address the effects of such transmitter ISI compensation. As example receivers in this document incorporate DFE, no further receiver duobinary (or other partial response code) processing will be shown.

Channelization

Physical transport channel limitations have been seen and addressed before, albeit at far lower data rates, during the efforts to provide high speed digital services over the legacy copper wire infrastructure of the telephony network. For DSL at its desired 3 Megabit data rate, a propagating signal wavelength was several hundred meters, which correlated strongly with the typical spacing of wire stubs, splices, and insulation abrasions seen in the field. Thus, an uncompensated frequency response for a typical copper telephony signal path would exhibit numerous notches and slopes caused by reflective interference among those anomalies, dissipative attenuation from degraded wires and insulation, and intrusive noise from sources such as AM radio transmitters.

Ultimately, multichannel frequency domain channelization was used to constrain the effect of those legacy transport issues. One commonly deployed Asymmetric Digital Subscriber Line (ADSL) solution, for example, partitioned the approximate 1 MHz of available transport medium bandwidth into 4.3125 kHz channels. Each channel was then independently tested for attenuation and signal-to-noise ratio, with different data throughput rates assigned to each channel depending on those test results. Thus, a channel frequency coinciding with a frequency response notch or significant external noise source would not be used, while other channels not presenting those issues could be used at full capacity. Unfortunately, the generation and detection of such a high channel count protocol relies on the availability of inexpensive digital signal processing solutions, and such technology has scaled in performance over time by perhaps a factor of ten, versus the approximate factor of 100,000 data rate increase in the present application.

Thus, although the present channel attenuation issues suggest a broadband approach may be useful, the conventional high-channel-count embodiment methods known to the art are incompatible with the anticipated data rate. A new approach specifically designed for high speed processing will be described.

Broadband Duobinary ENRZ

[Hormati III] gives examples of several embodiments combining ENRZ signaling with an additional serial transmission encoding such as Duobinary, utilizing multiple frequency-domain channels. Those examples and teachings are incorporated by reference herein, in their entirety for all purposes.

FIG. 1 is a block diagram of a further receiver embodiment that will be used to illustrate the following examples. In the receiver of FIG. 1, four communications wires w0-w3 carry a mixture of baseband and broadband signals; a first frequency channel is at baseband, i.e. comparable to a typical wire communication channel known in the art. The second frequency channel is herein called the “carrier”, “carrier-modulated”, or “broadband” channel, and is composed of ENRZ+duobinary signaling modulating a sinusoidal carrier, chosen to minimize the frequency overlap between spectral components of the baseband and of the carrier channel.

As in [Hormati III], a carrier frequency of 37.5 GHz is assumed. Both baseband and carrier channels run at a signaling rate of 37.5 Gsymbols/second, with a first set of three data bits being transported over the four wires of the baseband channel, and a second set of three data bits being transported over the same four wires using the carrier channel.

Other embodiments are known in which the baseband signaling rate differs from the carrier signaling rate, and/or differ from the carrier frequency. Generally speaking, however, there are implementation advantages in keeping these relationships fixed, often being expressed as ratios of small integer values, as in the 1:1:1 example provided above. As one example of such advantage, a receiver embodiment may then maintain a single local oscillator clock derived from one such received signal, and then derive the other necessary receive clocks from it through known phase lock or delay lock methods.

Filters 110 and 115 separate the received signals into a broadband component including a set of carrier-modulated symbols of a carrier-modulated codeword and a baseband component including a set of baseband symbols of a baseband codeword. For descriptive simplicity, FIG. 1 includes a High-pass filter 110 to generate the carrier-modulated symbols and a Low-pass filter 115 to generate the baseband symbols, although in practice band-pass or band-rejection filters may also be applicable. Following the baseband signal flow from the output of filter 115, so-called Multi-Input Comparators (MICs) 130 as per [Holden I] and [Ulrich I] perform weighted linear combinations of the various wire signals represented as baseband symbols, producing outputs which detect the individual subchannel data outputs of the ENRZ code. MICs 130 may additionally perform a slicing or time and amplitude sampling or measurement of each subchannel data output under control of a sampling clock provided by Clock/Data Recovery (CDR) subsystem 165.

As shown in FIG. 1, different system considerations may include different synchronization sources for CDR 165. In a first embodiment (1), a data-sampling clock is derived from data transitions of the detected subchannel data itself, using methods as described in [Hormati IV]. In a second embodiment (2), a subchannel of the ENRZ baseband signal provides a synchronization source, either by dedication to a periodic clock signal, or by augmentation of the transition density of a data stream to guarantee sufficient edge information to maintain clock synchronization. A third embodiment (3) uses a transmitted clock or synchronization signal distinct from the ENRZ data to synchronize the CDR subsystem, which may be transmitted from the transmitter to the receiver via a dedicated differential pair of wires.

As with the baseband data path, the broadband data detection path incorporates MIC-demodulation circuits 120, samplers 127, and CDR 150 functions. However, as broadband encoded data is modulated onto a carrier, simple data detection cannot be performed without addressing the carrier signal as well.

As is well understood in the art, a signal modulated on a carrier may be mixed with a demodulations signal provided by e.g., a local oscillator, to return the carrier-modulated signals to baseband (a heterodyne receiver,) or data detection may be timed relative to not only data sampling rate, but also relative to the carrier rate (a synchronous detector.) In FIG. 1, such an operation is performed by MIC-Demodulation circuits 120, as will be subsequently described. The demodulation signal may be generated by frequency multiplier 160, which in this example offered without limitation derives a carrier frequency reference from a sampling clock signal generated by baseband CDR 165. The detected subchannel information is processed using a Low Pass Filter 125, to remove residual artifacts of the heterodyne or synchronous detection operation of MIC-demodulation circuits 120. Carrier CDR subsystem 150 produces a data-sampling clock appropriate for the demodulated broadband channel data sampling 127. Depending on the particular demodulation method used by 120, Demodulation clock manager 140 may provide a demodulation signal as generated by frequency multiplier 160, a data sampling clock as provided by 150, or a combination of clocks for demodulation.

Alternative embodiments may synchronize a receiver clock to the carrier and derive other sampling clocks from that derived reference source, may synchronize a receiver clock to a detected data stream and derive other sampling clocks from that derived reference source, or utilize a combination of said methods. Synchronization may utilize a local voltage controlled oscillator (VCO) or voltage controlled delay in a phase-locked or delay-locked loop generating a local clock signal. Alternatively, synchronization may rely upon utilizing a phase comparator result that configures a phase interpolator or adjustable delay to modify the phase of a local clock signal.

Synchronously-Switched Multi-Input Demodulating Comparator

FIG. 2 is a schematic showing one embodiment of a linear mode ENRZ detector. Input signals w0, w1, w2, w3 are the received wire signals carrying the low-pass filtered (not shown) baseband symbols of the baseband codeword, and are connected to the transistors in accordance with a particular subchannel of the ENRZ code. That is, positive values of the subchannel vector indicate that the wire is connected to a transistor contributing to an output in a positive manner, while negative values indicate that the respective wire is connected to a transistor contributing to a negative portion of the output. Thus, in FIG. 2, the wires are connected to the transistors in accordance with the subchannel vector [1, −1, 1, −1], and the differential outputs QH and QL correspond to the result of the linear combination generated by the computation Q=(w0+w2)−(w1+w3)  (Eqn. 2)

As described in [Holden I], three instances of Eqn. 2 with different permutations of the input signals efficiently detect the three subchannel data outputs of the ENRZ code. Thus, the baseband detector 130 of FIG. 1 may be composed of three instances of the circuit of FIG. 2.

FIG. 3 is an embodiment of a synchronously-switched demodulating MIC circuit (also referred to herein as a “MIC-demodulation circuit”) implementing the same linear combination of the MIC in FIG. 2, in which the demodulation signal CK_d operating at the carrier frequency is used to gate operation of the MIC-demodulation circuit, allowing synchronous detection of carrier-modulated symbols of the carrier-modulated codeword. In a practical embodiment, CK_d will be phase locked to the carrier frequency provided by frequency multiplier 160 in FIG. 1. As shown in FIG. 3, the MIC-demodulation circuits obtains a set of carrier-modulated symbols of a carrier-modulated codeword. Each carrier-modulated symbol may be received via a respective wire of a plurality of wires of a multi-wire bus, and may have been high-pass (or band-pass) filtered prior to being applied to a corresponding transistor of the set of transistors shown. Each transistor is connected to a respective output node of a pair of output nodes according to elements of a sub-channel vector. For example, in the example shown in FIG. 3, the set of transistors connected to wires [w0 w1 w2 w3] are connected to the pair of output nodes QL and QH according to the sub-channel vector [1 −1 1 −1], which corresponds to the second row of the H4 Hadamard matrix given in Eqn. 1. The sets of transistors making up the MICs for the other two sub-channels may be connected to pairs of output nodes according to the other sub-channel vectors in the H4 Hadamard matrix that are mutually orthogonal to the [1 −1 1 −1] sub-channel vector. In some further embodiments, each transistor in the set of transistors may apply a respective magnitude weight to the carrier-modulated symbol according to the sub-channel vector. Eqn. 3 below is one particular matrix that includes sub-channel vectors having various magnitudes. Magnitude weights may be applied e.g., according to relative transistor dimensions, current source magnitudes, and/or multiple equal-sized transistors connected in parallel. The conductivity of the set of transistors is synchronously switched, and is controlled according to the demodulation signal CK_d, which is operating at a frequency recovered from the carrier-modulated symbols to responsively generate a demodulated sub-channel data output as a linear combination performed according to Eqn. 2 of the set of carrier-modulated symbols, which forms a differential voltage on the pair of output nodes.

$\begin{matrix} \begin{bmatrix} 1 & 1 & 1 & 1 & 1 & 1 \\ 1 & {- 1} & 0 & 0 & 0 & 0 \\ 1 & 1 & {- 2} & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & {- 1} & 0 \\ 0 & 0 & 0 & 1 & 1 & {- 2} \\ 1 & 1 & 1 & {- 1} & {- 1} & {- 1} \end{bmatrix} & \left( {{Eqn}.\mspace{14mu} 3} \right) \end{matrix}$

FIG. 4 is a further embodiment of a MIC-demodulation ENRZ detector. The MIC-demodulation circuit operates in a dynamic mode, charging the node capacitance of the pair of output nodes QH and QL when a sampling clock CK operating at the symbol rate is low, and selectably providing a discharge path for those nodes through the set of input transistors to ground when CK is high. As the nodes discharge, a differential output signal corresponding to the result of Eqn. 2 is produced at output nodes QH and QL. In such an embodiment, the sampling clock simultaneously acts as the demodulation signal as the carrier-modulated symbols are demodulated and the linear combination is sliced according to the sampling clock. Such an embodiment may occur when the sampling clock rate and the carrier frequency are equal. In some alternative embodiments, as shown, synchronous switching may be achieved using a demodulation signal CK_d to periodically interrupt or gate the dynamic operation of the detector at a carrier frequency rate, allowing direct detection of signals modulating a carrier. In some such embodiments, the demodulation signal CK_d may be an integer multiple of the baseband symbol baud rate of sampling clock CK, e.g., 2×, 3×, etc. Alternatively, the demodulation signal may have a frequency that is a fraction of the sampling clock rate.

FIG. 5 is a full-wave variation of the half-wave synchronous detector of FIG. 3. As with the half-wave detector, synchronously-switched ENRZ detection is gated by action of demodulation signal CK_d+ and its inverse or complement CK_d− which are phase locked to the carrier frequency. In an alternative embodiment, each differential pair of transistors may be composed of an NMOS device and a PMOS device that both receive the same demodulation signal CK_d+. In this full-wave circuit variation, the contribution of each input to the pair of output nodes is steered on alternating half cycles of the clock to the inverting and non-inverting result outputs, effectively doubling the output signal and substantially reducing the need for output filtering. The steering is performed by a plurality of differential pairs of transistors connected to the pair of output nodes for selectably connecting each transistor of the set of transistors to a respective output node according to the demodulation signal and its inverse. As shown, the set of transistors receiving the carrier-modulated symbols are alternately connected between the pair of output nodes via respective differential pairs of transistors. For each sub-channel MIC, the differential pairs of transistors may be connected according to the respective sub-channel vector of the plurality of mutually orthogonal sub-channel vectors. In the case of FIG. 5, the differential pairs of transistors are connected to the pair of output nodes according to the sub-channel vector [1 −1 1 −1].

FIG. 6 illustrates a discrete full-wave variation of the discrete half-wave dynamic synchronous detector of FIG. 4. Sampling clock CK controls the dynamic charge/discharge operation of the circuit, while demodulation signal CK_d+ and its complement CK_d− gate detection to be synchronous with the carrier frequency, as described above.

In the examples of FIGS. 4 and 6, the timing of the two clock signals may be coordinated so as to be compatible with the dynamic circuit operation. Specifically, in FIG. 4 a pre-charge operation occurring when clock CK is low may precede each discharge or integration cycle occurring when clocks CK_d and CK are both high. In FIG. 6, a full cycle of clock CK (i.e. a pre-charge cycle and a discharge cycle) may occur for each half-cycle of CK_d. Alternatively, the integration time for the circuit may be long enough to encompass two or more half-cycles of CK_d. In a further alternative embodiment, the demodulation signal CK_d+/− may be an integer multiple of CK. In such embodiments, multiple cycles of CK_d essentially gate the signal into or out of the integration period. In such embodiments, the pair of output nodes are pre-charged according to the sampling clock, and the pair of output nodes are only discharged when the carrier-modulated symbols at the inputs of the transistors have the correct polarity. In the full-wave embodiment, the pair of output nodes are always discharged, but the connections of the set of transistors receiving the carrier-modulated symbols to the pair of output nodes is alternated according to the demodulation signal as the carrier-modulated symbols alternate according to the modulation.

Operation of the half-wave and full-wave synchronous detectors is illustrated in the waveforms of FIG. 7. As described in [Hormati III], a transmitter multiplies Transmit Data and a Carrier-rate clock to produce a carrier-modulated signal. A receiver locally generates a carrier-rate demodulation clock, which is combined with the received signal in a synchronous detector to produce a detectable receive data signal. The carrier-modulated signal is encoded into carrier-modulated symbols of a carrier-modulated codeword and transmitted over the wires of the multi-wire bus.

The illustrated Return-to-Zero (RTZ) waveform in FIG. 7 is one example of an output of a half-wave detector circuit, while the Full-wave waveform is one example of an output of a full-wave detector circuit. In some embodiments, both such outputs are low-pass filtered to aid in reconstituting the detected waveform and eliminating spurious signal artifacts. Each waveform may then be sampled in the center of the data interval, as illustrated by the vertical hash marks.

FIG. 8A is a block diagram illustrating a conventional multiple PLL approach to generating the necessary receive clocks for an embodiment such as shown in FIG. 1. Using information obtained directly or indirectly from the received data (which may include transmitted clocks via dedicated wires, a dedicated sub-channel, and/or enhanced edge transition density, eye width or edge measurement, etc.) appropriately-timed sampling clocks are generated to permit optimum sampling of detected baseband data. This is commonly known as Clock-Data Alignment or Clock-Data Recovery (CDA or CDR). In common embodiments, a PLL configuration is used in which a phase detector controls a Voltage Controlled Oscillator (VCO) so as to generate a local clock signal having the desired characteristics.

Separately, an independent PLL configuration generates a local clock signal aligned to the carrier frequency of the received carrier-demodulated data, and a sampling clock suitable for optimum sampling of carrier-demodulated data.

In an alternative embodiment, one or more of the necessary local clock signals may be derived from another clock signal. FIG. 8B illustrates a local clock signal created using a PLL locked to a reference derived from the Baseband Data. This may be facilitated by, as representative examples, the Baseband data incorporating a dedicated clock signal, or being augmented by a guaranteed density of edge transitions. This well-controlled and stable local clock may then be used to generate other local clock signals, either by well-known frequency division or multiplication (e.g., using frequency multiplier 160 of FIG. 1), or simply by adjusting the relative phase of the derived clock using a phase interpolator or adjustable delay element (e.g., using offset correction element 140). For example, the local clock signal derived from baseband data may have the same frequency as the carrier frequency, and may thus be phase-adjusted using a phase detector and phase interpolator to generate the demodulation signal. Further, if the data rate of the demodulated sub-channel data is equal to the data rate of the baseband data, the clock derived from the baseband data may drive generation of the sampling clock for the demodulated sub-channel data, and may be phase-shifted accordingly. In some embodiments, the carrier frequency is different than the data rate of the baseband data, and thus the sampling clock generated from the baseband data may be multiplied/divided (not shown) to generate a demodulation signal having the carrier frequency (as is the case shown in FIG. 1). The demodulation signal may then be phase-detected and phase-shifted to be aligned to the carrier-modulated symbols. FIG. 8B shows two such derived clocks, each being phase-adjusted by a separate phase detector and phase interpolator. In some embodiments, the phase-adjustments are made to compensate for differences in the signal paths for the baseband/carrier signals, as they undergo different filtering, etc.

In some embodiments, hybrid clock generation embodiments are also possible, including embodiments utilizing secondary or slave PLLs that produce a second local clock that is derived from a first local clock generated as described above. In such a system configuration, the secondary PLL may have different lock characteristics than the primary PLL, allowing independent optimization of characteristics such as lock time, free-running drift, jitter, etc.

FIG. 9 is a flowchart of a method 900, in accordance with some embodiments. As shown, method 900 includes obtaining 902 a set of carrier-modulated symbols of a carrier-modulated codeword, each carrier-modulated symbol received via a respective wire of a plurality of wires of a multi-wire bus. Each carrier-modulated symbol of the set of carrier-modulated symbols is applied 904 to a corresponding transistor of a set of transistors, the set of transistors connected to a pair of output nodes according to a sub-channel vector of a plurality of sub-channel vectors. A demodulation signal CK_d is recovered 906 from the set of carrier-modulated symbols. A demodulated sub-channel data output is generated 908 as a differential voltage on the pair of output nodes based on a linear combination of the set of carrier-modulated symbols by controlling a conductivity of the set of transistors according to the demodulation signal CK_d.

In some embodiments, controlling the conductivity of the set of transistors includes selectably enabling a current source according to the demodulation signal CK_d, as shown in FIGS. 3 and 4. In such embodiments, the demodulated sub-channel data output is a return-to-zero (RTZ) signal, as illustrated in FIG. 7.

In some embodiments, controlling the conductivity further includes alternately connecting each transistor of the set of transistors between the pair of output nodes according to the demodulation signal. In such embodiments, each transistor circuit is alternately connected between the pair of output nodes using a respective differential pair of transistors connected to the pair of output nodes according to the sub-channel vector. In some embodiments, the respective differential pair of transistors receives the demodulation signal and a complement of the demodulation signal and is composed of same-type transistors (e.g., NMOS or PMOS only), while alternative embodiments may include differential pairs of transistors that include an NMOS and a PMOS transistor both receiving the demodulation signal CK_d. Such a configuration is illustrated in FIGS. 5 and 6, as the differential pairs receiving the demodulation signal CK_d + and a complement of the demodulation signal CK_d− alternate the connection of the corresponding transistor connected to the carrier-modulated symbols between each output node in accordance with the polarity of the carrier-modulated symbols.

In some embodiments, the conductivity of each transistor of the set of transistors is further controlled by a symbol value of the applied carrier-modulated symbol. In such embodiments, the amount of current drawn through each transistor is proportional to the symbol value applied at each transistor. Signal amplitudes in one particular embodiment are:

-   -   500 mV center,     -   500+180=680 mV (+1)     -   500−60=440 mV (−1/3)     -   500−60=440 mV (−1/3)     -   500−60=440 mV (−1/3)

where a symbol of magnitude ‘1’ corresponds to a 180 mV deviation from the 500 mV center voltage and a symbol of magnitude ‘1/3’ corresponds to a 60 mV deviation from the 500 mV center voltage.

In some embodiments, the method further includes pre-charging the pair of output nodes in response to a sampling clock, and wherein the conductivity of the set of transistors is further controlled according to the sampling clock. Such embodiments are referred to above as “discrete” or “dynamic” MIC-demodulation circuits.

In some embodiments, the differential voltage on the pair of output nodes is formed by drawing currents through impedance elements connected to the pair of output nodes. In some embodiments, the impedance elements may be resistors connected between a power supply and the pair of output nodes to control a voltage drop across the resistors. The differential amount of current drawn through the resistors will form a differential voltage output on the pair of output nodes.

In some embodiments, the method further includes low-pass filtering the demodulated sub-channel data output.

In some embodiments, obtaining the set of carrier-modulated symbols includes high-pass filtering a superposition codeword comprising the set of carrier-modulated symbols of the carrier-modulated codeword and a set of baseband symbols of a baseband codeword.

In some embodiments, the sub-channel vector is part of a plurality of mutually orthogonal sub-channel vectors that compose rows of an orthogonal matrix. In some such embodiments, the orthogonal matrix is a Hadamard matrix.

In some embodiments, the demodulation signal has an equal rate as a sampling clock associated with a baud rate of the data streams. In alternative embodiments, the demodulation signal has a differing rate than the sampling clock. In some such embodiments, the demodulation signal may be an integer multiple of the sampling clock, and may initiate multiple discharge periods in a single unit interval. Alternatively, the demodulation signal may be a fraction of the sampling clock. In such embodiments, the sampling clock may initiate multiple pre-charge/discharge cycles in a half cycle of the demodulation signal. In some embodiments, the demodulation signal is generated by multiplying the sampling clock using a frequency multiplier. Alternatively, the demodulation signal may be generated by dividing the sampling clock using a frequency divider. 

The invention claimed is:
 1. A method comprising: obtaining a set of carrier-modulated symbols of a carrier-modulated codeword, each carrier-modulated symbol received via a respective wire of a plurality of wires of a multi-wire bus; applying each carrier-modulated symbol of the set of carrier-modulated symbols to a corresponding transistor of a set of transistors, the set of transistors further connected to a pair of output nodes according to a sub-channel vector of a plurality of mutually orthogonal sub-channel vectors; recovering a demodulation signal from the carrier-modulated symbols; and generating a demodulated sub-channel data output as a differential voltage on the pair of output nodes based on a linear combination of the set of carrier-modulated symbols by controlling conductivity of the set of transistors according to the demodulation signal.
 2. The method of claim 1, wherein controlling the conductivity of the set of transistors comprises selectably enabling a current source according to the demodulation signal.
 3. The method of claim 2, wherein the demodulated sub-channel data output is a return-to-zero (RTZ) signal.
 4. The method of claim 1, wherein controlling the conductivity of the set of transistors further comprises alternately connecting each transistor of the set of transistors between the pair of output nodes according to the demodulation signal.
 5. The method of claim 4, wherein each transistor is alternately connected between the pair of output nodes using a respective differential pair of transistors connected to the pair of output nodes according to the sub-channel vector, the respective differential pair of transistors receiving the demodulation signal and a complement of the demodulation signal.
 6. The method of claim 1, wherein the conductivity of each transistor of the set of transistors is further controlled by a symbol value of the applied carrier-modulated symbol to draw an amount of current through each transistor that is proportional to the symbol value.
 7. The method of claim 1, further comprising pre-charging the pair of output nodes in response to a sampling clock, and wherein the conductivity of the set of transistors is further controlled according to the sampling clock.
 8. The method of claim 1, wherein the differential voltage on the pair of output nodes is formed by drawing currents through impedance elements connected to the pair of output nodes.
 9. The method of claim 1, further comprising low-pass filtering the demodulated sub-channel data output.
 10. The method of claim 1, wherein obtaining the set of carrier-modulated symbols comprises high-pass filtering a superposition codeword comprising the set of carrier-modulated symbols of the carrier-modulated codeword and a set of baseband symbols of a baseband codeword.
 11. An apparatus comprising: a plurality of wires of a multi-wire bus configured to obtain a set of carrier-modulated symbols of a carrier-modulated codeword, each carrier-modulated symbol received via a respective wire of the plurality of wires of the multi-wire bus; a set of transistors connected to a pair of output nodes according to a sub-channel vector of a plurality of mutually orthogonal sub-channel vectors, each transistor of the set of transistors receiving a respective carrier-modulated symbol of the set of carrier-modulated symbols; and a demodulation circuit configured to generate a demodulated sub-channel data output as a differential voltage on the pair of output nodes based on a linear combination of the set of carrier-modulated symbols by controlling conductivity of the set of transistors according to a demodulation signal operating at a frequency recovered from the carrier-modulated symbols.
 12. The apparatus of claim 11, wherein the demodulation circuit comprises a selectably-enabled current source controlled by the demodulation signal.
 13. The apparatus of claim 11, wherein the demodulated sub-channel data output is a return-to-zero (RTZ) signal.
 14. The apparatus of claim 11, wherein the demodulation circuit is configured to control the conductivity by alternately connecting each transistor of the set of transistors between the pair of output nodes according to the demodulation signal.
 15. The apparatus of claim 14, wherein each transistor of the set of transistors is alternately connected between the pair of output nodes via a respective differential pair of transistors connected to the pair of output nodes according to the sub-channel vector, the respective differential pair of transistors receiving the demodulation signal and a complement of the demodulation signal.
 16. The apparatus of claim 11, wherein the conductivity of each transistor of the set of transistors is further controlled by a symbol value of the applied carrier-modulated symbol to draw an amount of current through each transistor that is proportional to the symbol value.
 17. The apparatus of claim 11, further comprising a pre-charging pair of transistors configured to pre-charge the pair of output nodes in response to a sampling clock, and wherein the demodulation circuit further comprises a discharge transistor configured to control conductivity of the set of transistors according to the sampling clock.
 18. The apparatus of claim 11, further comprising a pair of impedance elements connected to the pair of output nodes, the impedance elements configured to form the differential voltage on the pair of output nodes by drawing currents through the impedance elements.
 19. The apparatus of claim 11, further comprising a low-pass filter configured to filter the demodulated sub-channel data output.
 20. The apparatus of claim 11, further comprising a high-pass filter configured to generate the set of carrier-modulated symbols by filtering a superposition codeword comprising the set of carrier-modulated symbols of the carrier-modulated codeword and a set of baseband symbols of a baseband codeword. 